1. Field of the Invention
The present invention relates to a combustion control device for an internal combustion engine provided with a plurality of fuel injection modes to be driven selectively changing the fuel injection mode, the combustion control device being so provided as to surely judge deterioration of a state of combustion in the internal combustion engine according to a fuel injection mode currently chosen from those fuel injection modes.
2. Description of the Prior Art
A lean-burn engine, which is one of the internal combustion engines, is driven at an air-fuel ratio leaner than the theoretical mixture ratio, thereby to improve rate of fuel consumption. A direct injection type gasoline engine is an engine which injects fuel directly into its combustion chamber, thereby to improve fuel consumption and reduce harmful exhaust gas components.
The direct injection type gasoline engine injects fuel, for example, from a fuel injection valve directly into a cavity provided at the top of a piston, thereby to produce an air-fuel mixture having an air-fuel ratio close to the theoretical mixture ratio around an ignition plug in accordance with ignition timing. Thus, in the case of the direct injection type gasoline engine, ignition is ensured even when an air-fuel mixture having an overall lean air-fuel ratio is supplied. Accordingly, it is possible to reduce discharge of CO and HC. Further, it is possible to largely improve fuel consumption at the time of idling and light-load driving. Furthermore, it is possible to improve acceleration and deceleration response, since, unlike a manifold fuel injection type engine, the direct injection type gasoline engine is free from a lag in fuel feeding due to an intake pipe, when injection quantity is increased or decreased.
The direct injection type gasoline engine has, however, such a problem that when a large quantity of fuel is injected, for example, at the time of heavy-load driving, an air-fuel ratio becomes over-rich in the vicinity of the ignition plug, which causes misfires. In order to solve the problem, for example, Japanese Patent Preliminary Publication No. Hei 7-102976 discloses a technique of driving an internal combustion engine in a compression stroke injection mode in which fuel is injected mainly in a compression stroke, or in an intake stroke injection mode in which fuel is injected mainly in an intake stroke.
The compression stroke injection mode is a mode in which fuel is injected into a cavity (for example, in the form of a basin portion or a recessed portion) provided at the top of a piston during the compression stroke, thereby to produce an air-fuel mixture having a air-fuel ratio (ratio by weight of air to fuel) close to the theoretical mixture ratio locally around the ignition plug and in the cavity. It is desirable as an fuel injection mode for light-load driving. The intake stroke injection mode is a mode in which fuel is injected outside the aforementioned cavity during the intake stroke, thereby to produce a homogeneous air-fuel mixture having a lean air-fuel ratio throughout the combustion chamber. It is desirable as an fuel injection mode for heavy-load driving.
In each of those fuel injection modes, fuel injection timing is determined as follows: First, injection-valve opening time is determined based on fuel pressure and fuel injection quantity. Then, injection end timing is so determined that fuel injection may end during the intake stroke or the compression stroke. Then, injection start timing is determined based on the aforementioned injection end timing and the injection-valve opening time. Particularly in the compression stroke injection mode, the injection end timing and the injection start timing are so determined, considering also a time required for fuel atomization, that fuel in the cavity may be completely atomized at the time of ignition, in order to prevent incomplete combustion.
When the aforementioned fuel injection modes are adopted with a view to improving fuel consumption, it is desirable that driving regions in which the internal combustion engine is driven at a lean air-fuel ratio in the aforementioned fuel injection modes are determined to be as large as possible. Such driving regions are determined depending on load and engine speed of the internal combustion engine. However, when the driving regions in which the internal engine is driven in the aforementioned compression stroke injection mode or in the intake stroke injection mode are determined to be large, deterioration of a state of combustion may be produced.
For example, when fuel injection quantity during the compression stroke increases with an increase in load on the internal combustion engine, an air-fuel ratio may become over-rich, which causes misfires. Inversely, when an air-fuel ratio becomes too lean during the intake stroke, incomplete combustion (insufficient combustion) may be produced. When deterioration of combustion such as misfires, incomplete combustion (insufficient combustion) or the like is produced, it is necessary to detect such a state quickly and rectify conditions of combustion. Conditions of combustion are rectified, for example, by retarding the ignition timing (in the case where the air-fuel ratio has become over-rich), or by enriching the air-fuel ratio (in the case where the air-fuel ratio has become lean during the intake stroke).
In the case of conventional internal combustion engines, cycle-by-cycle variation of combustion is judged from angular acceleration or angular acceleration deviation of the engine based on the same criterion, regardless of the fuel injection mode. Therefore, a critical lean level of an air-fuel ratio in each of the aforementioned fuel injection modes can not be surely judged. Deterioration of combustion such as misfires, incomplete combustion (insufficient combustion) or the like varies according as the fuel injection mode is the compression stroke injection mode, the intake stroke injection mode or a normal injection mode in which the internal combustion engine is driven at the theoretical mixture ratio. Nevertheless, in the case of conventional internal combustion engines, quantity of variation of combustion is judged based on the same criterion, so that deterioration of combustion, which varies depending on the fuel injection mode, can not be surely judged.